1. Field of the Invention
Embodiments of the invention relate to the field of ion implantation. More particularly, the present invention relates to an apparatus and method for controlling beam current uniformity for low energy beam transport in an ion implanter.
2. Discussion of Related Art
Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium are doped to create p-type regions in the substrate. An ion implanter used for this purpose generally includes an ion source chamber which generates ions of a particular species, a series of beam line components to control the ion beam and a platen to secure the wafer that receives the ion beam. These components are housed in a vacuum environment to prevent contamination and dispersion of the ion beam. The beam line components may include a series of electrodes to extract the ions from the source chamber, a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio are able to travel through the analyzer, and a corrector or collimator magnet to provide a ribbon beam which is orthogonally directed to a substrate with respect to the ion beam to implant the ions into the crystal lattice. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy.
The depth of ion implantation into the substrate is based on the ion implant energy and ion mass. Smaller electronic device sizes require high beam current densities implanted at low energy levels (for example ≦2 keV). Typically, low energy ion beams diverge as they travel through an ion implanter because of beam “blow-up” which is due to space charge effect. Space charge effect is where positively charged ions in the beam repel each other causing the beam to diverge from the beam line path. One way to neutralize this space charge effect is to introduce a plasma into the beam path such that the number of positively and negatively charged particles within the beam space are the same. This may be accomplished by introducing a neutral gas into the beam line so that beam-plasma can be generated via beam-potential interaction with the neutral gas. The extent of space charge neutralization may be controlled by the introduction and composition of the particular type of the neutral gas into the beam line. For example, one method positions a nozzle near the extraction region and injects water vapor to the beam to counteract the space charge effect for neutralizing boron ion beams. However, this method may not sufficiently improve the ion beam uniformity.